Can I add additional LibreOffice/Hunspell dictionaries to Sigil for languages other than the default languages?
If so, what LibreOffice/Hunspell version(s) does Sigil support and how do I add them to Sigil?

That should be possible. You can also use the ones from OpenOffice as far as I know.

I just tested Sigil with the LibreOffice 3.5 dictionaries (from \share\extensions) and they work indeed fine. I assumed that I'd need to change a configuration file, but all I had to do was copy them to Sigil's \dictionaries folder.